checking for an existing cookie

Hi,

I have the following code:
   if (Response.Cookies["888"] == null)
            {

                aCookie = new HttpCookie("888");
                aCookie.Value = System.Guid.NewGuid().ToString();                
                aCookie.Expires = DateTime.Now.AddDays(7);
                Response.Cookies.Add(aCookie);

I've just used a dummy name 888 for now but I'm trying to check to see if a cookie exists and if not create it.

However the code above always gets stepped over, even when the cookie shouldn't exist.

Am I doing the correct check?

Thanks